We are investigating the hormonal and nonhormonal regulation of maternal behavior in the rat, the ontogeny of maternal behavior, and the neural and neuropharmacological mechanisms underlying this behavior. The role of estrogen and progesterone interaction during pregnancy on the onset of maternal behavior ill be studied, the neural site of action of estrogen, and the possible neurotransmitter mediation of estrogen action will be investigated. The role of pubertal hormones and of olfactory stimulation (olfactory bulb and vomeronasal organ) on male and female maternal responsiveness and changes during ontogeny will be investigated.